wouldnt we have found the broken isles before now?

Pretty much this. The only continent we didn't 'know about' was Pandaria. Or rather, we didn't know specifically where it was.
We've always known where the broken isles, northrend, zandalar, etc. are. There's just no point, from a gameplay perspective, in adding them to the map until you can actually go there

I didn't find the right solution from the Internet.

References:
http://www.mmo-champion.com/threads/1961...sles-before-now

Product Promotional Video Example